Douglas-fir beetle

1Principal Entomologist and Project Leader (retired), Forestry Sciences Laboratory, USDA Forest Service, Moscow, ID; 2Forest Entomologist, USDA Forest Service, Forest Health Protection, Northern Region, Coeur d’Alene, ID. Douglas-fir beetle galleries are distinctive, having eggs laid individually in niches alternating on opposite sides of the gallery. The Douglas-fir beetle (Dendroctonus pseudotsugae Hopkins) occurs throughout the extensive range of its primary host, Douglas-fir, in western North America (Figure 1). In areas of the Northwest, it also infests downed western larch but not live larch trees. Douglas-fir is unique among North American trees by its adaptation to environments varying from mild maritime along the Pacific coast to seasonally drier and colder elevations inland up to 10,000 ft. as in the Escalante Mountains, Utah and the Sierras in Mexico. Botanists recognize two varieties of Douglas-fir. The coastal variety, Pseudotsuga menziesii var. menziesii (Mirb.) Franco, attains heights of over 300 ft. The inland form, var. glauca (Biessn) Franco, is of lesser size, with recorded tree heights ranging from 85 ft. in southern Utah to 140 ft. in Idaho. Douglas-fir stands become disjunct toward its southern distribution (Figure 1) and this isolation has resulted in a genetic divergence in Mexico indicative of a third variety and the development of a subspecies of the beetle, Dendroctonus pseudotsugae barragani Furniss (Furniss 2001), whereas the subspecies Dentroctonus pseudotsugae pseudotsugae Hopkins occurs northward in the United States and Canada.